This application is related to commonly-assigned U.S. patent application Ser. No. 11/584,095, filed Oct. 20, 2006; now U.S. Pat. No. 7,716,971, issued on May 18, 2010; and U.S. patent application Ser. No. 11/589,579, filed Oct. 30, 2006; now U.S. Pat. No. 7,677,089, issued on Mar. 16, 2010.
The present invention relates to a protection system of a powerplant machine; and more particularly to a method of determining when to test an overspeed protection system of the powerplant machine.
An overspeed condition occurs after the speed of a shaft on a powerplant machine exceeds a specified range. During the overspeed condition, a powerplant machine typically experiences severe mechanical and thermal stresses that can cause a catastrophic failure.
Generally, the powerplant machine is equipped with protection systems, which attempt to reduce the effects of an overspeed condition. A governor system generally serves as the primary line of protection. Upon detecting an overspeed condition, the governor attempts to decrease the speed of the shaft. There may also be a secondary, or independent line, of protection—an overspeed protection system. Typically, the overspeed protection system incorporates mechanical, electrical, and software components to safeguard the powerplant machine. An overspeed protection system protects the powerplant machine by initiating an emergency shutdown (commonly called a trip) during an overspeed event.
Powerplant operators periodically test the overspeed protection system to determine if the system is functioning properly. The interval between testing is commonly on a fixed schedule. For example, on a yearly basis.
Prior to testing the overspeed protection system, the powerplant machine is customarily operating in a full-speed-no-load (FSNL) condition. FSNL is a condition when the powerplant machine is at a normal operating speed and is not exporting energy to a load. An overspeed test typically involves manually raising the speed of a shaft above the normal operating range. For example, but not limiting of, during an overspeed test operators may raise the speed of the shaft to 110% of the normal operating speed; thereafter the overspeed protection system should trip the powerplant machine.
There are a few problems with current methods of overspeed testing. Current methods use a fixed interval for the periodic testing of the overspeed protection system. The current methods do not determine if the interval between testing may be extended because of operating conditions of the powerplant machine and associated turbine systems. These systems may include, but are not limited to: speed detection; emergency stop valve; lube oil system; hydraulic system; trip oil system; emergency trip; or protective trip. These systems are collectively referred to as “turbine systems”, hereinafter.
Manually adjusting the shaft speed may introduce high thermal transients. A trip at a speed near or above the normal operating speed can introduce large mechanical, electrical, and thermal stresses on the components of the powerplant machine. These stresses decrease the maintenance interval. Moreover, after a trip, a re-start of the powerplant machine is required, which delays the exporting of energy. In addition, the current overspeed testing methods typically require that the powerplant machine operate at FSNL. This does not generate revenue but consumes fuel and electricity. These problems drive powerplant machine operators to avoid manual speed adjustments, trips, FSNL operation, and overspeed testing.
For the foregoing reasons, there is a desire for a method of testing an overspeed protection system that reduces the chance of a trip. The method should provide a condition based system, or the like, which may monitor the turbine systems. Based on the monitoring, the method should determine whether a test of the overspeed protection system should be performed. The method should be adaptable to a wide variety powerplant machines, including those integrated with others powerplant machines.